This.
A common soldier and an orc might know or are pretty sure to know that wizards do exist, the probability of this is dependent on how common wizards are in the specific campaign world.
They also might know that wizards can cast spells. And that they also might know that some wizards can cast spells which lead to fiery explosions, which could injure or kill more than one enemy but that is not necessarily true, some might think wizards only have the power to divine things or make people fall asleep or charm them etc..
What they absolutely do not know is :
- Whether it is a fireball that hit them or e.g. minute meteor or one part of a meteor swarm etc.
- What range, diameter or damage potential a fireball has got (e.g. how far they need to spread out to prevent the wizard from hitting them all)
- If any party member when they approach them is a wizard at all, unless e.g. every wizard wears a pointy head and a star spangled robe always.
- Even if they know a party member is a wizard they eventually know nothing about what capabilities he might have
So they would have to act in a "spread out " just in case of the party having a evocation type wizard with the knowledge to cast fireball, although this might be giving them tactical disadvantages if said party is only made up of other classes than wizards (Think 4 orcs ganking 1 party "fighter" or 4 party "fighters" ganking 1 orc).